Systems and methods for optical full-field transmission using photonic integration

ABSTRACT

An optical full-field transmitter for an optical communications network includes a primary laser source configured to provide a narrow spectral linewidth for a primary laser signal, and a first intensity modulator in communication with a first amplitude data source. The first intensity modulator is configured to output a first amplitude-modulated optical signal from the laser signal. The transmitter further includes a first phase modulator in communication with a first phase data source and the first amplitude-modulated optical signal. The first phase modulator is configured to output a first two-stage full-field optical signal. The primary laser source has a structure based on a III-V compound semiconductor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 62/837,087, filed Apr. 22, 2019, which isincorporated herein by reference in its entirety.

BACKGROUND

The field of the disclosure relates generally to fiber communicationnetworks, and more particularly, to optical networks implementingoptical signal modulation.

Many of today's telecommunications networks include an access networkthrough which end user subscribers connect to a service provider.Bandwidth requirements for delivering high-speed data-intensiveapplications and services through the access network are rapidlyincreasing to meet growing consumer demands, and particularly in thecontinuously-developing fields of high-definition video-on-demand, cloudservices, Internet of Things (IoT), and “Big Data.” At present, datadelivery over the access network is growing by gigabits (Gb)/second forresidential subscribers, and multi-Gb/s for business subscribers, and isexpected to reach multi-Gb/s for typical users.

To accommodate such high demand in a scalable manner, a variety ofoptical network architectures have been proposed and implemented in thefield. In one example, Time Domain Multiple Access (TDMA) techniquesprovide a dynamic time slot assignment to respective users, and therebyenable multiple users to share the same passive network infrastructure.TDMA, however, is limited in its ability to provide a guaranteed datarate per user. To overcome this limitation, wavelength divisionmultiplexing (WDM) techniques have been implemented to improve capacityper user and provide an additional degree of flexibility through dynamicwavelength assignment. Conventional WDM technology, however, is limitedby low receiver sensitivity, and by few options available to upgrade andscale the technology.

Present access networks are often based on passive optical network (PON)access technologies, which have become a dominant system architecture tomeet growing high capacity demand from end users. However, a currentconsolidation trend in the field has led many network operators toreduce the number of optical line terminals (OLTs) in the PON.Furthermore, optical access networks using conventional signaltransmission technology, such as intensity modulation with directdetection (IM-DD), are reaching a saturation point. As data rateincreases beyond 10 Gb/s (10G), IM-DD signal transmission suffers frominefficient spectrum utilization, and is susceptible to fiberimpairments such as chromatic dispersion (CD) and polarization modedispersion (PMD).

As PON systems evolve towards 100 Gb/s and higher data rates, coherenttechnology is increasingly being seen as a solution for optical accessnetworks, due to its superior performance and vast potential overconventional signal transmission technologies. In particular, incomparison with IM-DD systems, which suffer from limited modulationbandwidth, short transmission distance, and poor received sensitivity,coherent technology systems achieve significantly higher receiversensitivity, inherent frequency selectivity, and linear field detectioncapability that enables full compensation of linear channel impairments.Coherent technology supports exceptionally high data throughput overlong reach (e.g., >50 km), and both increases the overall capacity andextends the power budget for WDM-PON systems.

Long distance transmission using coherent technology, however, requireselaborate post-processing, including signal equalizations and carrierrecovery, to adjust for impairments experienced along the transmissionpathway, thereby presenting significant challenges by significantlyincreasing system complexity. Thus, despite the benefits afforded bycoherent technology, these benefits are obtained at a greater financialcost due to the high complexity of coherent transceivers and receivers.The cost challenges of coherent technology is significantly magnifiedwith respect to implementation in PON-based optical access networks.

For example, in the downlink of conventional PONs, the complexity limitson the transceiver in the OLT, such as at the headend, central office,and/or hub, are less stringent than the limits placed on a receiver inan optical network unit (ONU), since the cost of the OLT transceiver,which sends and receives data to and from multiple ONUs, is shared byall end users supported in the respective network. In contrast, the costof each ONU is driven, and likely borne solely, by the respective enduser. Accordingly, lowering the cost and complexity of the coherentcomponent technology will more significantly impact the ONU than theOLT. For this reason, the complexity and high cost of conventionalcoherent transceivers has been limited to point-to-point (P2P)applications, which may employ one ONU per OLT, and not implemented inpoint-to-multipoint (P2MP) applications, which typically employ a numberof ONUs for each OLT.

The overall cost of conventional coherent systems is dominated by thecomplex optical and opto-electronic components of the coherenttransceivers, such as high-performance tunable lasers andlocal-oscillators (LOs), high-speed modulators, digital signalprocessing (DSP) chips, polarization optics, etc. Accordingly, there isa need in the industry for further innovations to reduce the cost ofcoherent optical components in the coherent access network. Several suchinnovations have been recently introduced by the present inventors.Novel systems and methods implementing coherent optical injectionlocking (COIL)-based architectures, for example, are described ingreater detail in U.S. Pat. Nos. 9,912,409 and 10,200,123 to the sameinventors, both of which are incorporated by reference herein in theirentireties.

Accordingly, it is desirable to further improve upon these novel systemsand methods to develop even further savings in the cost, power, andfootprint of efficient coherent transceivers to meet the uniquerequirements of access networks, while also enabling implementation ofhybrid PONs that may employ a mix of WDM and TDMA techniques.

BRIEF SUMMARY

In an embodiment, an optical full-field transmitter is provided for anoptical communications network. The transmitter includes a primary lasersource configured to provide a narrow spectral linewidth for a primarylaser signal, and a first intensity modulator in communication with afirst amplitude data source. The first intensity modulator is configuredto output a first amplitude-modulated optical signal from the lasersignal. The transmitter further includes a first phase modulator incommunication with a first phase data source and the firstamplitude-modulated optical signal. The first phase modulator isconfigured to output a first two-stage full-field optical signal. Theprimary laser source has a structure based on a III-V compoundsemiconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of a dual-polarization coherentoptical injection locking transmitter implementing full-field modulationbased on amplitude and phase.

FIGS. 2A-B are graphical illustrations depicting an operationalprinciple of the coherent optical injection locking transmitter depictedin FIG. 1.

FIG. 3 is a schematic illustration depicting an exemplarydual-polarization optical full-field transmitter.

FIG. 4 is a schematic illustration depicting an exemplarysingle-polarization optical full-field transmitter.

FIG. 5 is a schematic illustration depicting an alternativedual-polarization optical full-field transmitter.

FIG. 6 is a schematic illustration depicting an alternativesingle-polarization optical full-field transmitter.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of this disclosure. These featuresare believed to be applicable in a wide variety of systems including oneor more embodiments of this disclosure. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the embodimentsdisclosed herein.

DETAILED DESCRIPTION

In the following specification and the claims, reference will be made toa number of terms, which shall be defined to have the followingmeanings.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

As used herein, the terms “processor” and “computer” and related terms,e.g., “processing device”, “computing device”, and “controller” are notlimited to just those integrated circuits referred to in the art as acomputer, but broadly refers to a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit (ASIC), and other programmable circuits, and these terms areused interchangeably herein. In the embodiments described herein, memorymay include, but is not limited to, a computer-readable medium, such asa random access memory (RAM), and a computer-readable nonvolatilemedium, such as flash memory. Alternatively, a floppy disk, a compactdisc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or adigital versatile disc (DVD) may also be used. Also, in the embodimentsdescribed herein, additional input channels may be, but are not limitedto, computer peripherals associated with an operator interface such as amouse and a keyboard. Alternatively, other computer peripherals may alsobe used that may include, for example, but not be limited to, a scanner.Furthermore, in the exemplary embodiment, additional output channels mayinclude, but not be limited to, an operator interface monitor.

Further, as used herein, the terms “software” and “firmware” areinterchangeable, and include computer program storage in memory forexecution by personal computers, workstations, clients, and servers.

As used herein, the term “non-transitory computer-readable media” isintended to be representative of any tangible computer-based deviceimplemented in any method or technology for short-term and long-termstorage of information, such as, computer-readable instructions, datastructures, program modules and sub-modules, or other data in anydevice. Therefore, the methods described herein may be encoded asexecutable instructions embodied in a tangible, non-transitory, computerreadable medium, including, without limitation, a storage device and amemory device. Such instructions, when executed by a processor, causethe processor to perform at least a portion of the methods describedherein. Moreover, as used herein, the term “non-transitorycomputer-readable media” includes all tangible, computer-readable media,including, without limitation, non-transitory computer storage devices,including, without limitation, volatile and nonvolatile media, andremovable and non-removable media such as a firmware, physical andvirtual storage, CD-ROMs, DVDs, and any other digital source such as anetwork or the Internet, as well as yet to be developed digital means,with the sole exception being a transitory, propagating signal.

Furthermore, as used herein, the term “real-time” refers to at least oneof the time of occurrence of the associated events, the time ofmeasurement and collection of predetermined data, the time for acomputing device (e.g., a processor) to process the data, and the timeof a system response to the events and the environment. In theembodiments described herein, these activities and events occursubstantially instantaneously.

As used herein, “modem termination system” (MTS) refers to a terminationunit including one or more of an Optical Network Terminal (ONT), anoptical line termination (OLT), a network termination unit, a satellitetermination unit, a cable modem termination system (CMTS), and/or othertermination systems which may be individually or collectively referredto as an MTS.

As used herein, “modem” refers to a modem device, including one or morea cable modem (CM), a satellite modem, an optical network unit (ONU), aDSL unit, etc., which may be individually or collectively referred to asmodems.

As described herein, a “PON” generally refers to a passive opticalnetwork or system having components labeled according to known namingconventions of similar elements that are used in conventional PONsystems. For example, an OLT may be implemented at an aggregation point,such as a headend/hub, and multiple ONUs may be disposed and operable ata plurality of end user, customer premises, or subscriber locations.Accordingly, an “uplink transmission” refers to an upstream transmissionfrom an end user to a headend/hub, and a “downlink transmission” refersto a downstream transmission from a headend/hub to the end user, whichmay be presumed to be generally broadcasting continuously (unless in apower saving mode, or the like).

In the following embodiments, innovative systems and methods aredescribed for implementing photonics integration into the coherenttechnology-based optical access network, as described above. The presentembodiments also further improve upon the innovative optical full-fieldtransmitter (OFFT) architectures and techniques described in greaterdetail in co-pending U.S. application Ser. No. 16/711,293, filed Dec.11, 2019, by the same inventors. The subject matter of this co-pendingapplication is also incorporated by reference herein in its entirety.The embodiments described herein introduce photonic integration intothese co-pending technological innovations, and thereby realize stillfurther cost and space savings that satisfy the increasing demand forinexpensive and efficient coherent transceivers in the access networkparadigm.

FIG. 1 is a schematic illustration of a dual-polarization coherentoptical injection locking (COIL) transmitter 100 implementing full-fieldmodulation based on amplitude and phase. In an embodiment, transmitter100 includes a master laser 102, a first slave laser 104, and a secondslave laser 106. Master laser 102 may, for example, be an externalcavity laser (ECL), and first and second lasers 104, 106 may beFabry-Perot (FP) lasers in the COIL implementation thereof. In thisexample, first and second slave lasers 104, 106 are dedicated tomodulating separate first and second polarizations (e.g., X- andY-polarizations), respectively, of a dual-polarization signal.

More particularly, in operation of transmitter 100, the laser signalfrom master laser 102 is distributed to first slave laser 104 and secondslave laser 106 by an optical coupler 108 in optical communication withmaster laser 102, first slave laser 104, and second slave laser 106.First slave laser 104 modulates, for the first single-polarization(e.g., X-polarization), a first amplitude data signal 110. A first phasedata signal 112 is modulated by a first phase modulator 114 (e.g.,external to first slave laser 104), which is in optical communicationwith a first optical circulator 116 disposed between first slave laser104 and first phase modulator 114, and also between first slave laser104 and optical coupler 108 (e.g., a three-port optical coupler, in thisexample).

That is, to transmit signals with both intensity and phase information,first slave laser is directly modulated for amplitude modulation offirst amplitude data signal 110, which is combined with first phase datasignal 112 modulated by first external phase modulator 114 for phasemodulation and rotation. In this example, master laser 102 is presumedto be a high-quality ECL utilized to injection-lock first slave laser104. First optical circulator 116 therefore functions to not only routeboth the master laser beam to first slave laser 104, but also tofunction to route the amplitude-modulated signal from first slave laser104 to first phase modulator 114 that modulates the phase information offirst phase data signal 112. By synthesizing the amplitude-modulationinformation and phase-modulation information of the first polarizationinto a first synthesized optical signal 118, full-field opticaltransmission may be achieved for the first single-polarization signal.

In a similar manner, second slave laser 106 modulates, for the secondsingle-polarization (e.g., Y-polarization), a second amplitude datasignal 120, and a second phase data signal 122 is modulated by a secondphase modulator 124 in optical communication with a second opticalcirculator 126. Second optical circulator 126 is similarly disposedbetween second slave laser 106 and second phase modulator 124, and alsobetween second slave laser 106 and optical coupler 108. A secondsynthesized optical signal 128 is achieved by synthesizing theamplitude-modulation information and phase-modulation information of thesecond polarization, thereby realizing full-field optical transmissionfor the second single-polarization signal as well. Dual-polarizationOFFT operation is achieved for transmitter 100 by combining first andsecond synthesized optical signals 118, 128 with apolarization-beam-combiner (PBC) 130, which outputs a dual-polarizationmodulated optical output signal 132.

Systems and methods according to transmitter 100 are thus capable ofreducing the cost for both the laser and the modulator. Additionally,the improved architecture that combines a phase modulator (e.g., phasemodulators 114, 124) with a directly-modulated COIL laser (e.g., slavelasers 104, 106, respectively) significantly reduces both the opticalinsertion loss and the modulation loss seen by conventional utilizationof parallel Mach-Zehnder Modulators (MZMs) in a conventional coherentI/Q modulator. The improved architecture of transmitter 100 effectivelyeliminates the need for the MZM for coherent modulation.

FIGS. 2A-B are graphical illustrations depicting an operationalprinciple 200 of COIL transmitter 100, FIG. 1. More particularly,operational principle 200 achieves two-stage, full-field transmitterinjection locking according to an amplitude modulation stage 202 and aphase modulation stage 204. In the embodiment depicted in FIGS. 2A and2B, amplitude modulation stage 202 and phase modulation stage 204 areillustrated in the polar coordinate plane for illustrative purposes. Atamplitude modulation stage 202, for example, transmitter 100 implementsinjection locking and amplitude modulation to obtain anamplitude-modulated COIL signal 206 (e.g., seen at circulator 116 or126, FIG. 1). At phase modulation stage 204, transmitter 100 appliesphase modulation to achieve full-field optical modulation of afull-field signal 208 (e.g., first or second synthesized optical signals118, 128, FIG. 1). Since, according to operational principle 200, theamplitude and phase modulation remain naturally orthogonal to eachother, implementation of both amplitude modulation stage 202 and phasemodulation stage 204 together may optionally render the additionalimplementation of a bias control (e.g., in a separate, or third stage)unnecessary.

In comparison with conventional MZM-based modulators, which are known toexperience significantly large insertion and modulation losses,transmitter 100 and operational principle 200 demonstrate significantadvantages over such conventional techniques by achieving asignificantly higher output power from the gain components present inthe lasers. Indeed, in comparison with such conventional techniques,operational principle 200 demonstrates that the full-field transmittermay be implemented using only one COIL set of master/slave lasers (i.e.,as opposed to a dual-polarization architecture, as illustrated in FIG.1), with no bias or phase shift control needed. The structuralconfiguration of transmitter 100 though, would not exclude the use of anexternal phase modulator, if so desired.

The improved design and operation of transmitter 100 though, maynevertheless result in an additional power consumption, due at least inpart from the integrated optical circulator, which requires theadditional polarization rotation components described above with respectto FIG. 2B. The following embodiments though, describe and illustrateseveral photonics integration solutions that address these particularchallenges. These further solutions present alternative architecturesand techniques that leverage state-of-art photonics integrationtechnology based on silicon photonics (SiPho) and/or Indium Phosphide(InP) photonics. According to these several embodiments, the structuraland operational design of the OFFT is significantly improved, andtherefore of particular advantageous use in a PON, a coherent PON(CPON), a P2P coherent optical access network, or a P2MP coherentnetwork.

SiPho-Based OFFT

Due to the large-scale production capability of the CMOS siliconindustry, SiPho-based technologies are seen to be promising forproviding highly integrated and low-cost components and systems for thefuture and near-future optical communication networks. Monolithicintegration of SiPho technologies has already demonstrated generallygood performance when implemented with respect to components such asmodulators, photodetectors, polarization beam splitters/combiners, andother passive devices. However, this field is still lacking for apractical silicon-based light source solution.

The need for such practical light source solutions has driven recentresearch in the field toward the investigation of heterogeneousintegration of III-V compound semiconductors, such as InP, on silicon.Typically, unstructured InP dies are bonded, epitaxial layers down, on asilicon-on-insulator (SoI) waveguide circuit wafer, after which the InPgrowth substrate is removed and the III-V epitaxial layer is patterned.With this approach, integrated high-performance ECLs having a widetuning range and narrow spectral linewidth have been achieved. Today,commercially-available ECLs typically include multiple discretecomponents, such as two etalon filters, an InP gain chip, an end mirror,an optical isolator, and a few micro-lenses, etc. In contrast, anintegrated solution for a hybrid laser is described below with respectto FIG. 3, which implements InP on a silicon platform, and thus achievesa significant reduction to both the fabrication and packaging costs.

FIG. 3 is a schematic illustration depicting an exemplarydual-polarization OFFT 300. In an exemplary embodiment, OFFT 300includes an integrated Si/InP hybrid laser 302, which thereby representsa heterogeneous integration of OFFT on an SoI platform. Moreparticularly, OFFT 300 advantageously utilizes the photonics integrationon the SoI platform to achieve heterogeneous integration on a singlechip. It may be noted here that the phrase “heterogeneous integration”should not be confused with the heterogeneous multiplying of differentsignals of different types onto a single fiber strand for transmission.Although these different technologies are complementary, the term“heterogeneous” is applied differently to the two applications.

As depicted in FIG. 3, a heterogeneous OFFT photonics integration designon an SoI platform may be integrated on a single chip. Thisheterogeneous design thus further enables a significant simplificationto the architecture of, for example, the COIL portion of transmitter100, FIG. 1. That is, use of a high quality ECL integrated on thesilicon enables achievement of the modified-OFFT-design-on-SoI. Morespecifically, by utilizing an integrated, or hybrid, ECL as thehigh-quality narrow-linewidth light source of hybrid laser 302, the twoslave lasers and two optical circulators shown in transmitter 100, FIG.1, may be rendered unnecessary in the simplified architecture of OFFT300. Additionally, the fabrication process for OFFT is simplified incomparison with that of transmitter 100, since transmitter 100 requiresinclusion of the slave lasers and optical circulators, as well as anumber of polarization rotation waveguides, which collectively increasethe complexity of the overall component. In contrast, the simplifieddesign of OFFT 300 renders these additional components unnecessary.

In an exemplary embodiment, hybrid laser 302 may be a Si/InP hybrid ECLhaving (i) a passive SoI waveguide portion, and (ii) an active III-Vportion, such as InP (not separately shown). Such a hybrid ECL may, forexample, be fabricated by bonding the III-V active chip onto the SOIsubstrate, and then patterning the active region. Modal transfer betweenthe III-V active layer and silicon passive layer may then be achievedusing taper structures. Ring resonators in the silicon passive portionmay be provided to enable single mode selection and wavelength tuningbased on a thermal-optical effect, e.g., using upper metal heaters.

In exemplary operation of OFFT 300, the output of laser 302 is splitinto first path 304 and second path 306 by a polarization beam splitterrotator (PBSR) 308. In an embodiment, PBSR 308 may be a tapered siliconwaveguide-based structure. In further operation of OFFT 300, the signalof first path 304 is amplitude-modulated by a first silicon amplifiermodulator 310, which may, for example, include a silicon MZMconstruction in communication with a first amplitude data source 312,and then phase-modulated by a first phase modulator 314 in communicationwith a first phase data source 316. In a similar manner, the signal ofsecond path 306 is amplitude-modulated by a second silicon amplifiermodulator 318 (e.g., also a silicon MZM construction, in this example)in communication with a second amplitude data source 320, and thenphase-modulated by a second phase modulator 322 in communication with asecond phase data source 324.

In the exemplary embodiment, both the silicon-based MZM amplitudemodulators 310, 318, and phase modulators 314, 322, may becarrier-depletion type modulators based on a free carrier plasmadispersion effect, to enable high modulation efficiencies, low drivingvoltages, high extinction ratios, and wide bandwidths. After thedual-modulation operations, the two-stage modulated signals from firstand second paths 304, 306 are combined by a PBC 326 to realizepolarization division multiplexing for a combined modulated opticalsignal output 328. Combined modulated optical signal output 328 may thenbe coupled off-chip by way of a mode size converter (not shown).

Accordingly, in comparison with a COIL-based OFFT design (e.g.,transmitter 100, FIG. 1), the fabrication process of OFFT 300 is asignificantly simplified such that heterogeneous integration is onlyrequired for the ECL portion thereof. The COIL-based design illustratedin FIG. 1, on the other hand, would require heterogeneous or hybridintegration for both of the two slave lasers and the two opticalcirculators, which could render such devices more complex and moreexpensive. The fabrication of OFFT 300 may be even further simplifiedthrough monolithic integration of PBSR 308 with silicon MZMs 310, 318and phase modulators 314, 322. Thus, the majority of OFFT 300 may befabricated using standard CMOS fabrication processes, since only hybridECL 302 would require heterogeneous bonding of the InP gain media ontothe SoI waveguides.

The exemplary embodiment of OFFT 300 depicted in FIG. 3 is describedabove utilizing silicon MZMs for intensity modulation, this example isprovided by way of illustration, and not in a limiting sense. Althoughstate-of-art silicon MZMs, due to their wide modulation bandwidth andhigh extinction ratio, may be generally effective for an OFFT accordingto the embodiment depicted in FIG. 3, MZMs are also known to occupy alarge footprint area on a chip (usually several millimeters in length),and also to introduce high optical loss. Nevertheless, both amplitudemodulation and phase modulation are necessary for the OFFT to generatethe intensity and phase modulated signal. Accordingly, a simplified OFFTarchitecture, which eliminates the need for an MZM, is described furtherbelow with respect to FIG. 4.

FIG. 4 is a schematic illustration depicting an exemplarysingle-polarization OFFT 400. OFFT 400 is similar to OFFT 300, andincludes a laser 402 that may also be an Si/InP heterogeneouslyintegrated ECL, in the exemplary embodiment. OFFT 400 differs though,from OFFT 300, in that OFFT 400 directly modulates ECL 402 to generatethe amplitude modulation signal. That is, according to the advantageousconfiguration of OFFT 400, instead of providing a separate amplitudemodulator, laser 402 is directly in communication with an amplitude datasource 404, and a 10 Gb/s non-return-to-zero (NRZ) signal may begenerated by directly modulating the current of integrated ECL 402. Theoperation of OFFT 400 is then otherwise similar to that of OFFT 300, inthat the intensity-modulated signal from laser 402 is thenphase-modulated by a phase modulator 406 that is in communication with aphase data source 408 (e.g., to add phase information through phasemodulator 406), to produce a modulated optical signal output 410 havingboth amplitude and phase modulation.

This direct modulation technique for integrated ECL 402 thus furthersimplifies the fabrication process of the OFFT, for example, incomparison with that of OFFT 300, FIG. 3, which integrates an externalMZM with the laser. OFFT 400, therefore, represents a comparatively morecost-efficient approach. For ease of explanation, OFFT 400 is depictedin FIG. 4 as using a directly-modulated ECL for a single polarization.

Thus, according to the advantageous techniques described herein for OFFT400, the on-chip optical loss and large device footprint of an OFFTaccording FIG. 3 may be significantly reduced by removing the need forsilicon MZMs. However, some trade-off between the embodiments depictedin FIGS. 3 and 4 occurs for implementing a single-polarization caseversus a dual-polarization case. Additionally, the modulation bandwidthof the directly-modulated laser of OFFT 400 is considered to be morelimited in comparison to that of the MZM of OFFT 300. Accordingly, itmay be more desirable to implement devices according to OFFT 400 forlower-cost applications, such as in the ONUs of a coherent PON, anddevices according to OFFT 300 in the OLTs of the coherent PON, and/orP2P coherent access networks, which are less sensitive to cost, for thereasons discussed above.

InP Photonics-Based OFFT

In addition to the hybrid embodiments described above, the followingembodiments demonstrate how InP, apart from the hybrid implementationswith silicon, offers another material platform for state-of-the-artperformance in optoelectronic devices, and more particularly suchdevices and systems operating in the 1300-1600 nm wavelength window. InPis a direct bandgap III-V compound semiconductor material; theembodiments described further herein advantageously leverage InPphotonics integrated circuitry to enable monolithic integration of lasersources with other system or device components, such as modulators,amplifiers, multiplexers, and detectors, in wafer-scale processes andfabrication. An OFFT fabricated on an InP platform is described furtherbelow with respect to FIG. 5.

FIG. 5 is a schematic illustration depicting an alternativedual-polarization OFFT 500. In an exemplary embodiment, OFFT 500 issimilar to OFFT 300, FIG. 3, in its general architectural configurationand operation. OFFT 500 differs from OFFT 300 though, in that OFFT 500advantageously leverages the monolithic integration capability of InPsuch that all of the respective components of OFFT 500 may be fabricateddirectly on an InP substrate without requiring hybrid/heterogeneousintegration.

More particularly, in the exemplary embodiment, OFFT 500 includes alaser 502 as the light source thereof. Laser 502 may, for example,advantageously utilize one or more of a distributed feedback (DFB)laser, a distributed Bragg reflector (DBR) laser, and a discrete mode(DM) laser as the light source. These types of lasers may beparticularly desirable for low-cost implementations, since each of theselaser types is able to achieve single mode operation with relativelynarrow linewidth, and while maintaining wavelength tuning capability upto a few nanometers.

In exemplary operation of OFFT 500, the output from laser 502 is splitinto first path 504 and second path 506 by a PBSR 508. The signal offirst path 504 is amplitude-modulated by a first InP amplifier modulator510, which may, for example, include an MZM or an electro-absorptionmodulator (EAM) in communication with a first amplitude data source 512.The amplitude-modulated signal along first path 504 is thenphase-modulated by a first InP phase modulator 514 in communication witha first phase data source 516. In a similar manner, the signal of secondpath 506 is amplitude-modulated by a second InP amplifier modulator 518in communication with a second amplitude data source 520, and thenphase-modulated by a second InP phase modulator 522 in communicationwith a second phase data source 524. The amplitude- and phase-modulatedsignals from first and second paths 504, 506 are combined by a PBC 526to realize polarization division multiplexing for a combined modulatedoptical signal output 528, which may be coupled off-chip by way of amode size converter, similar to the silicon-based embodiment of OFFT300, FIG. 3.

Also similar to OFFT 300, FIG. 3, because DFB/DBR/DM lasers are capableof high-speed direct modulation, the innovative principles of OFFT 500may also be adapted to a single-polarization counterpart, similar toSiPho-based OFFT 400, FIG. 4, to effectively replace the MZM/EAMintensity modulators (e.g., first and second InP amplitude modulators510, 518) by directly modulating the laser (e.g., laser 502). Anexemplary InP single-polarization OFFT configuration is describedfurther below with respect to FIG. 6.

FIG. 6 is a schematic illustration depicting an alternativesingle-polarization OFFT 600. In an exemplary embodiment, OFFT 600 issimilar to OFFT 400, FIG. 4, in general architecture and operation, butdiffers in that OFFT 600 advantageously leverages the monolithicintegration InP capability of OFFT 500, FIG. 5, but for asingle-polarization configuration that eliminates the need for separateintensity/amplitude modulators.

In an exemplary embodiment, OFFT 600 includes a laser 602, which may bea DFB, DBR, or DM laser, similar to laser 502, FIG. 5. Laser 602 differsthough, from laser 502, in that laser 602 is directly modulated by OFFT600. That is, laser 602 is directly in communication with an amplitudedata source 604, and generates an intensity-modulated signal from laser602 that is then phase-modulated by a phase modulator 606, which itselfis in communication with a phase data source 608. A modulated opticalsignal output 610, having both amplitude and phase modulation, isgenerated after operation of phase modulator 606.

According to the advantageous configuration of OFFT 600, the directmodulation of laser 602 enables not only a much simpler fabricationprocess, but also eliminates the extra insertion loss that would havebeen introduced by an external MZM/EAM (e.g., used in OFFT 500, FIG. 5).Nevertheless, the implementation of direct amplitude modulation (i.e.,at laser 602), followed by phase modulation (i.e., through phasemodulator 606) forms the OFFT (i.e., OFFT 600), as explained above withrespect to FIGS. 2A-B.

Similar to the SiPho embodiment described above with respect to FIG. 4,an OFFT according to the configuration depicted in FIG. 6 requires onlyone laser source, plus one phase modulator, for OFFT operation. Alsosimilar to the SiPho embodiment though, the InP embodiment of OFFT 600may be subject to trade-offs with respect to single-polarizationapplications, and/or according to modulation bandwidth limitations oflaser 602. Nevertheless, implementation of the principles of OFFT 600enables significant reductions to both the on-chip optical loss, as wellas the device footprint. The ability to reduce the size of the chip is asignificant advantage achieved by the present InP platform embodimentsover conventional devices, given that an InP wafer size is typicallylimited to approximately four inches.

According to the innovative embodiments described herein, uniquesystems, apparatuses, and methods are provided for several OFFT designson SiPho and InP platforms to reduce either or both of the cost andfootprint of an optoelectronic device based on these technologyplatforms. These innovative embodiments herein are particularlyadvantageous for use in coherent optical access network applications,including but not limited to PONs and P2P communication networks. Inexemplary embodiments, the technological innovations described hereinyield significant improvements to both dual-polarization andsingle-polarization implementations.

Exemplary embodiments of optical communication systems, devices, andmethods are described above in detail. The systems and methods of thisdisclosure though, are not limited to only the specific embodimentsdescribed herein, but rather, the components and/or steps of theirimplementation may be utilized independently and separately from othercomponents and/or steps described herein. Additionally, the exemplaryembodiments can be implemented and utilized in connection with otheraccess networks utilizing fiber and coaxial transmission at the end userstage.

The embodiments described herein may be further implemented, for examplewith respect optical communication networks utilizing a DOCSIS protocol,and also with respect to one or more systems utilizing protocols such asEPON, RFoG, GPON, or Satellite Internet Protocol, without departing fromthe scope of the embodiments herein. The present embodiments aretherefore particularly useful for communication systems configured foruse in existing 4G and 5G networks, and also for new radio (NR) andfuture generation networks that utilize such communication protocols, inwhole or in part.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, such illustrativetechniques are for convenience only. In accordance with the principlesof the disclosure, a particular feature shown in a drawing may bereferenced and/or claimed in combination with features of the otherdrawings.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor or controller, suchas a general purpose central processing unit (CPU), a graphicsprocessing unit (GPU), a microcontroller, a reduced instruction setcomputer (RISC) processor, an application specific integrated circuit(ASIC), a programmable logic circuit (PLC), a field programmable gatearray (FPGA), a DSP device, and/or any other circuit or processorcapable of executing the functions described herein. The processesdescribed herein may be encoded as executable instructions embodied in acomputer readable medium, including, without limitation, a storagedevice and/or a memory device. Such instructions, when executed by aprocessor, cause the processor to perform at least a portion of themethods described herein. The above examples are exemplary only, andthus are not intended to limit in any way the definition and/or meaningof the term “processor.”

This written description uses examples to disclose the embodiments,including the best mode, and also enables a person skilled in the art topractice the embodiments, including the make and use of any devices orsystems and the performance of any incorporated methods. The patentablescope of the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal language of the claims.

The invention claimed is:
 1. An optical full-field transmitter for anoptical communications network, the transmitter comprising: a primarylaser source configured to provide a narrow spectral linewidth for aprimary laser signal; a first intensity modulator in communication witha first amplitude data source, the first intensity modulator configuredto implement amplitude modulation to output a first amplitude-modulatedoptical signal from the primary laser signal; and a first phasemodulator in communication with a first phase data source and the firstamplitude-modulated optical signal, the first phase modulator configuredto implement phase modulation on the first amplitude-modulated opticalsignal to output a first two-stage full-field optical signal, whereinthe primary laser source has a structure based on a III-V compound,wherein the amplitude modulation by the first intensity modulator isorthogonal to the phase modulation by the first phase modulator, andwherein the primary laser source, the first intensity modulator, and thefirst phase modulator are integrated together.
 2. The transmitter ofclaim 1, wherein the III-V compound comprises indium phosphide (InP). 3.The transmitter of claim 2, wherein the primary laser source comprisesan integrated silicon (Si)/InP hybrid laser.
 4. The transmitter of claim3, wherein the integrated Si/InP hybrid laser comprises a heterogeneousintegration on a silicon-on-insulator (SoI) waveguide circuit wafer. 5.The transmitter of claim 4, wherein the integrated Si/InP hybrid laserfurther comprises a passive SoI waveguide portion and an active InPportion.
 6. The transmitter of claim 3, wherein the first phasemodulator comprises a silicon-based construction.
 7. The transmitter ofclaim 6, wherein the integrated Si/InP hybrid laser includes the firstintensity modulator.
 8. The transmitter of claim 7, wherein the firsttwo-stage full-field optical signal is a single-polarization signal. 9.The transmitter of claim 6, wherein the first intensity modulator isdisposed in a first optical path between the integrated Si/InP hybridlaser and the first phase modulator.
 10. The transmitter of claim 9,wherein the first intensity modulator comprises a silicon-basedMach-Zehnder modulator construction.
 11. The transmitter of claim 9,further comprising: a polarization beam splitter rotator (PBSR)configured to receive the primary laser signal from the primary lasersource and route the primary laser signal to the first optical path andto a second optical path different from the first optical path; a secondintensity modulator disposed along the second optical path opposite theintegrated Si/InP hybrid laser with respect to the PBSR, the secondintensity modulator in communication with a second amplitude data sourceand configured to output a second amplitude-modulated optical signalfrom the primary laser signal; a second phase modulator in communicationwith a second phase data source and the second amplitude-modulatedoptical signal, the second phase modulator configured to output a secondtwo-stage full-field optical signal; and a polarization beam combinerconfigured to receive the first and second two-stage full-field opticalsignals and output a dual-polarization optical signal.
 12. Thetransmitter of claim 2, wherein the primary laser source comprises anInP photonics-based laser formed on an InP substrate.
 13. Thetransmitter of claim 12, wherein the first phase modulator comprises anInP-based construction.
 14. The transmitter of claim 13, wherein the InPphotonics-based laser includes the first intensity modulator.
 15. Thetransmitter of claim 14, wherein the first two-stage full-field opticalsignal is a single-polarization signal.
 16. The transmitter of claim 13,wherein the first intensity modulator is disposed in a first opticalpath between the InP photonics-based laser and the first phasemodulator.
 17. The transmitter of claim 16, wherein the first intensitymodulator comprises an InP-based construction comprising at least one ofa Mach-Zehnder modulator and an electro-absorption modulator.
 18. Thetransmitter of claim 16, wherein the InP photonics-based laser, thefirst intensity modulator, and the first phase modulator aremonolithically integrated together.
 19. The transmitter of claim 16,further comprising: a polarization beam splitter rotator (PBSR)configured to receive the primary laser signal from the primary lasersource and route the primary laser signal to the first optical path andto a second optical path different from the first optical path; a secondintensity modulator disposed along the second optical path opposite theInP photonics-based laser with respect to the PBSR, the second intensitymodulator in communication with a second amplitude data source andconfigured to output a second amplitude-modulated optical signal fromthe primary laser signal; a second phase modulator in communication witha second phase data source and the second amplitude-modulated opticalsignal, the second phase modulator configured to output a secondtwo-stage full-field optical signal; and a polarization beam combinerconfigured to receive the first and second two-stage full-field opticalsignals and output a dual-polarization optical signal.
 20. Thetransmitter of claim 19, wherein the InP photonics-based laser, thePBSR, the first and second intensity modulators, the first and secondphase modulators, and the polarization beam combiner are monolithicallyintegrated together.